java - 我对类里面的 setter/getter 和返回感到困惑？

Question

我一直在自学编码、 getter 和返回值，以及如何在主程序中调用它们。为了尝试一下，我尝试编写一个程序来计算长途电话的费用，但是当我运行它时它就崩溃了，我知道这与类有关。

公平警告，我计算 AM/PM 的时间有点奇怪，但我尽力了。

我认为这必须是调用我的主代码中的计算——特别是 String weekday = call1.calculateweekday();和 int time = call1.calculatetime(); ，但我对编程非常陌生，我才刚刚开始学习这些术语和用途，所以我不知道是什么。我只知道，当我在主程序中移动这两行时，它会在此时中断。

package practice;

import java.util.Scanner;

class Call {
    String weekdayinput;
    String weekday;
    int hour;
    String ampm;
    int time;

    int calculatetime() {

        if (ampm.equals("pm") || ampm.equals("PM") || ampm.equals("Pm")) {

            time = hour + 1200;
        } else if (ampm.equals("am") || ampm.equals("AM") || ampm.equals("Am")) {

            time = hour;

        }

        else {
            System.out.println("You entered something either time or AM/PM incorrectly.");
        }

        return time;
    }

    String calculateweekday() {

        if (weekdayinput.equals("mo") || weekdayinput.equals("Mo") || weekdayinput.equals("MO")) {

            weekday = "Monday";
        }

        else if (weekdayinput.equals("tu") || weekdayinput.equals("Tu") || weekdayinput.equals("TU")) {

            weekday = "Tuesday";
        }

        else if (weekdayinput.equals("we") || weekdayinput.equals("We") || weekdayinput.equals("WE")) {

            weekday = "Wednesday";
        }

        else if (weekdayinput.equals("th") || weekdayinput.equals("Th") || weekdayinput.equals("TH")) {

            weekday = "Thursday";
        }

        else if (weekdayinput.equals("fr") || weekdayinput.equals("Fr") || weekdayinput.equals("FR")) {

            weekday = "Friday";
        }

        else if (weekdayinput.equals("sa") || weekdayinput.equals("Sa") || weekdayinput.equals("SA")) {

            weekday = "Saturday";
        }

        else if (weekdayinput.equals("su") || weekdayinput.equals("Su") || weekdayinput.equals("SU")) {

            weekday = "Sunday";
        }

        else {
            System.out.println("You entered your weekday incorrectly.");
        }

        return weekday;
    }

}

public class GettersandREturns {
    public static void main(String args[]) {

        Scanner input = new Scanner(System.in);
        Call call1 = new Call();

        String weekday = call1.calculateweekday();
        int time = call1.calculatetime();

        System.out.println("To calculate the cost per minute of your long-distance call, we'll need some information.");
        System.out.println(
                "What hour are you planning on making the call. Minutes aren't necessary. Please only enter the hour number. (ex. 8)");
        call1.hour = input.nextInt();
        input.hasNextLine();
        System.out.println("Is the call taking place AM or PM?");
        call1.ampm = input.nextLine();
        input.hasNextLine();
        System.out.println("And what day of the week is that? Please enter weekday with only first two letters. (ex. Fr");
        call1.weekdayinput = input.nextLine();



        if (time >= 8 && time <= 11 && !weekday.equals("Saturday") && !weekday.equals("Sunday")
                || time >= 1212 && time <= 1206 && !weekday.equals("Saturday") && !weekday.equals("Sunday"))

        {
            System.out.println("Your call will charge $4.50 a minute.");
        }

        else if (time == 12 && !weekday.equals("Saturday") && !weekday.equals("Sunday")
                || time >= 1 && time < 8 && !weekday.equals("Saturday") && !weekday.equals("Sunday")
                || time > 1206 && time <= 1211 && !weekday.equals("Saturday") && !weekday.equals("Sunday")) {

            System.out.println("Your call will charge $4.00 a minute.");

        }

        else if (weekday.equals("Saturday") || weekday.equals("Sunday")){

            System.out.println("Your call will charge $2.25 a minute.");
        }

        else {
            System.out.println("You must have entered something wrong!");
        }

    }
}

因此，我们的想法是，周一至周五上午 8:00 至下午 6:00 之间发起的任何调用均按每分钟 4.50 美元计费。周一至周五上午 8:00 之前或下午 6:00 之后开始的任何通话均按费率计费每分钟 4.00 美元。最后，任何在周六或周日调用的电话均按每分钟 2.25 美元的费率收费。

但是当我运行这个程序时，我在线程“main”java.lang.NullPointerException中得到异常在 Practice.Call.calculateweekday(GettersandREturns.java:32)在practice.GettersandREturns.main(GettersandREturns.java:82)

任何帮助将不胜感激。学习很难。

Answer 1

在主函数中，您将创建一个新调用并调用该调用的函数。

Call call1 = new Call();

    String weekday = call1.calculateweekday();
    int time = call1.calculatetime();

此时，如果您查看具有以下变量的 Call 类，您会收到错误

String weekdayinput;
String weekday;
int hour;
String ampm;
int time;

你会发现这些变量一开始并没有初始化，对于int来说默认值是0，对于String来说默认值是null(在编程的时候有时会遇到空指针异常)。只要您不尝试访问此变量，就不会禁止使用 null 值，而这正是您的函数所做的。

原因:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“boolean java.lang.String.equals(java.lang.Object)”

将是错误

 if (ampm.equals("pm") || ampm.equals("PM") || ampm.equals("Pm"))

将会抛出异常，因为您的 ampm 或您尝试访问的任何其他字符串此时为 null，并且您正在尝试将 null 字符串与字符串进行比较。

确保在将字符串与任何内容进行比较之前先对其进行初始化，例如与

String weekdayinput = "mo";
String weekday "mo";
int hour;
String ampm "am";
int time;

在请求输入后，您应该调用工作日和时间的函数。

 Scanner input = new Scanner(System.in);
    Call call1 = new Call();

    System.out.println("To calculate the cost per minute of your long-distance call, we'll need some information.");
    System.out.println(
            "What hour are you planning on making the call. Minutes aren't necessary. Please only enter the hour number. (ex. 8)");
    call1.hour = input.nextInt();
    input.hasNextLine();
    System.out.println("Is the call taking place AM or PM?");
    call1.ampm = input.nextLine();
    input.hasNextLine();
    System.out.println("And what day of the week is that? Please enter weekday with only first two letters. (ex. Fr");
    call1.weekdayinput = input.nextLine();
 String weekday = call1.calculateweekday();
    int time = call1.calculatetime();

作为旁注:您应该阅读关于可以替换这么多 if 语句的开关。